1. Field of the Invention
The present invention relates to printing apparatuses that convey sheets and perform printing.
2. Description of the Related Art Japanese Patent Laid-Open No. 2009-6655 discloses a printing apparatus that directly measures the speed of a sheet surface using a speed sensor to control the timing at which ink is ejected from print heads. FIG. 8 is a simplified diagram showing a printing apparatus disclosed in FIG. 25 of Japanese Patent Laid-Open No. 2009-6655. A roll of sheet 500 is conveyed by a conveying roller pair 501 on the upstream side and a conveying roller pair 502 on the downstream side and is subjected to printing by print heads 503. A speed sensor 504 (a laser Doppler sensor) that directly measures the moving speed of the sheet is disposed between the conveying roller pair 501 on the upstream side and the print heads 503. The timing of driving the print heads 503 is corrected based on the sheet conveyance speed measured by the speed sensor 504, thereby achieving high-quality printing.
In fields requiring mass printing, for example, printing labs, increasing printing speed while maintaining image quality is a problem. In addition, a demand for duplex printing, in which printing is performed on both surfaces of a sheet, is increasing because it enables production of photo books etc.
The apparatus disclosed in Japanese Patent Laid-Open No. 2009-6655 can sequentially print a plurality of images on one side of a continuous sheet. However, it is not designed to print on both surfaces of a sheet. In duplex printing, the first surface and second surface of the sheet, with which a conveying roller comes into contact, have different coefficients of friction. In particular, when ink is applied, the coefficient of friction of the sheet surface changes significantly. As a result, the slippage between the conveying roller and the sheet surface in printing on the second surface is different from that in preceding printing on the first surface, whereby the sheet conveyance condition is different even if the same driving force is applied. Therefore, if the same correction is performed in printing on the first surface and on the second surface with the method disclosed in Japanese Patent Laid-Open No. 2009-6655, the image on the second surface has a size different from the originally intended size. Thus, the images on the front and back sides have different sizes.
Furthermore, the laser Doppler sensor used in the apparatus disclosed in Japanese Patent Laid-Open No. 2009-6655 temporarily stores measured information, performs signal processing, and outputs the result, because of its measurement principle. Thus, delay in detection due to complicated signal processing may limit the speed, which may prevent high-speed real-time correction and make it difficult to increase the printing speed (moving speed of the sheet).